Data transducer head positioners employ both open and closed servo loop technologies in order to move the data transducer head among concentric data storage tracks of a rotating storage disk coated with magnetic storage media during track seeking operations, and to keep the data transducer head aligned with each selected data track during track following operations, when data is being read from or written to the data track being followed. The head positioner structure includes an actuator for translating electrical driving currents into motion to move the head back and forth in a general radial direction across the data storage surface, and to maintain the head in alignment with each selected track during track following operations.
In order to position the head, a driving amplifier converts position error signals into the driving currents which flow through the actuator coil. The position error signal may be developed by servo feedback information provided from a dedicated servo surface of the disk, or it may be provided on a sampled and held basis from servo sectors recorded in interleave fashion with data sectors in one or more of the concentric data tracks, or it may be derived from an external transducer, such as a polyphase optical transducer coupled to the actuator structure.
The object of the position error signal is to effectuate a correspondence between a nominal track centerline and the actual track centerline during track following operations; and, to keep the head moving along a radial seeking locus in accordance with a desired seeking profile during track seeking operations.
The assignee of the present invention has pioneered the development of fixed disk drives employing a mass balanced rotary actuator with a polyphase optical transducer having a microline scale closely coupled to the rotating arm of the actuator, and further having prerecorded servo correction information periodically sampled from the disk surface and held to correct the position information provided by the optical transducer. Examples of disk drives employing this technology are to be found in commonly assigned, or commonly owned and cross-licensed patents, including by way of example U.S. Pat. No. 4,396,959 (now U.S. Reissue Patent Re 32,075 and also see U.S. Pat. No. 4,920,434 based on same disclosure); U.S. Pat. Nos. 4,639,798; 4,639,863; 4,858,034; and U.S. patent application Ser. No. 07/192,353, filed on May 10, 1988, now U.S. Pat. No. 5,005,089.
While the head positioner servo loop architecture employed in the disk drives described in these patents has worked very well, as the need for a greater number of data tracks has continued to grow, the density of concentric data tracks with reference to a given small diameter data storage surface has become limited by a number of factors relating to spindle offset tolerances and other tolerances found within the disk drive.
A first limitation is related to spindle offset tolerances. In these examples of prior disk drives, the data storage disk(s) is mounted to a spindle by a clamping mechanism. The servo correction information provided on the disk storage surface is prerecorded at the factory by using the polyphase position transducer and other circuitry and programming to perform a servo writer function. The servo correction information is written when the head and disk assembly has reached a nominal operating temperature after a warm-up period.
Unfortunately, over time, the data storage disk may shift slightly with respect to the rotational axis of the spindle, leading to an offset characteristic. This offset may be due to repeated thermal cycles as the disk drive is turned on and off, or it may be due to application of a shock force, particularly likely if the drive is included within small, portable equipment, such as laptop computers which are carried about and subjected to jarring and dropping, etc.
The drawback of spindle offset is that data written on one circular locus deemed to be a data track centerline at one time may not be capable of being read later on if the disk develops an offset with respect to the disk spindle axis of rotation, leading to eccentricity of the original track centerline relative to the spindle axis of rotation. Also, writing data on a track of a disk having an offset may result in the loss of data earlier written on an adjacent concentric data track before the offset developed.
There have been a number of prior approaches to the solution of spindle offset, particularly with respect to removable disk packs. One example is provided in the Chick et al. U.S. Pat. No. 4,136,365 which teaches the use of two adjacent reference loci prerecorded with a phase coherent tri-bit pattern throughout their circumferential extent. The servo pattern is periodically accessed by the data transducer head during interruptions in data transfers, and thermal drift as well as offset is measured and recorded in memory for later use in correcting head position over data tracks. A scheme using an outer reference track, an inner reference track and interpolation between the two tracks is also suggested by the Chick et al. patent.
A sampled sector offset correction arrangement is disclosed in the Jacques et al. U.S. Pat. No. 4,135,217 which provided a number of embedded servo sectors, such as 24 sectors, within each data track. A microprocessor was used in the Jacques et al. disk drive to process samples of the servo information read from each sector in order to generate an offset prediction value for correcting head position. The drawback of these approaches has been the relative complexity needed to generate and apply an offset (eccentricity) error signal to the servo loop summing junction to correct for offset, particularly in the case of fixed disks.
Thus, one hitherto unsolved need has arisen for a more simplified, yet effective mechanism for compensating for spindle offset tolerances within a fixed disk drive.
A second limitation has been associated with electrical drifts within or related to the polyphase encoder. It is known to derive two electrical, phase related signals P1 and P2 from the polyphase position encoder which is mounted to the disk drive base and which has a moveable microline scale closely coupled to the drive's rotary actuator structure. The phase signals P1 and P2 resulting from relative transmission of light photons through the scale and reticle-masked photodetector array are most desirably nominally in phase quadrature (i.e. 90 degree phase lead/lag between P1 and P2). By use of arc tangent based algorithms or table lookups, it is practical to combine the P1/P2 sine/cosine values in order to derive e.g. 128 equal angle positions for each track, with 512 angle positions being equal to one complete optical cycle of each of the P1 and P2 phases (one cycle resolving radial head position for four adjacent data tracks). Thus, for the track extending between position zero and position 127, centerline is nominally at position 63. For the track extending from position 128 to position 255, centerline is nominally at position 191. For the track extending from position 256 to position 383, centerline is nominally at angle position 319. And, for the track extending from position 394 to 511, track centerline is nominally at angle position 457. (See FIG. 11 and discussion of encoder circle hereinafter).
When in quadrature relationship and after setup calibration procedures are completed, the P1 and P2 optical phase signals are most preferably close approximates of sine and cosine waves as the scale moves relative to the photodetector array. During such movement and when P1 amplitude is plotted on e.g. the vertical axis and when P2 amplitude is plotted on e.g. the horizontal axis, a circle lissajous results. This encoder circle or lissajous has been referred to as a "circle servo", which is a shorthand expression for describing this particular implementation and use of the P1 and P2 phase signals and resultant encoder circle.
Phase lead and lag of the phases P1 and P2 may be trimmed by rotational adjustment of the optical encoder assembly relative to the scale mounted to the rotary actuator as is taught for example by commonly assigned U.S. Pat. No. 4,647,769, the disclosure thereof being incorporated herein by reference. Also symmetry of the circle lissajous is established and promoted by providing highly collimated light energy. This may be achieved by placing the LED at right angles to the scale and reticle and using a mirror to redirect the light path toward the scale and reticle as was done for example in commonly owned and cross-licensed U.S. Pat. No. 4,703,176.
Most desirably, in order for the positional vernier of 128 positions per track to be realized, the full dynamic range of the analog to digital converter should be employed during the conversion of the P1 and P2 analog signals into digital values. Accordingly, the lissajous diameter should be made as large as possible without becoming saturated or distorted. However, the diameter of the P1/P2 circle lissajous is subject to variations, due primarily to changes in gap dimension between the scale and the reticle and also due to changes in light source output, drift of electrical component values and characteristics. Whereas in prior disk drive designs the scale was mounted on a very rigid scale mounting arm structure extending from the rotary actuator assembly, if the mass of the mounting arm structure is reduced in order to reduce power consumption requirements of the rotary actuator, and/or shorten track accessing times during seeking and resultant data throughput rates, it becomes increasingly difficult to maintain precisely the desired gap dimension between the scale and the reticle masking the photodetector array.
Commonly assigned U.S. Pat. No. 4,593,194 describes an optical encoder having digital gain compensation for controlling light intensity. As each photodetector was polled and its output digitized, the microcontroller adjusted the light source (LED) by a computed value. This prior approach required microcontroller activity for making the LED adjustment each time that a P1 or P2 phase was digitized. Also, because only two photodetectors were employed in the optical encoder array to provide the P1 and P2 phase signals, common mode rejection with consequent improved noise immunity was not available.
Also, when the line spacing of the scale is reduced in order to define more tracks which are closer together, the need for highly collimated light and minimization of penumbra effects is heightened.
Thus, a second hitherto unsolved need has remained for a more efficient method for an arrangement within an optical encoder enabling greater track densities to be realized and for periodic measurement of light source amplitude and readjustment if necessary within the polyphase optical encoder in order to maximize the undistorted diameter of the circle lissajous thereby providing maximized quantization dynamic range and better noise immunity than heretofore achieved by the prior art approaches.